The invention relates to a construction comprising an electronic power device, such as for example a high-power converter, an IGBT, a power-MOSFET, power diodes or combinations of power devices on a DCB ceramic. The DCB ceramic of the power device is cooled directly by a heat sink, in order to remove the dissipated power of the power device.
To reduce the thermal resistance between the power device and the electrical contact layer, it has been proposed in EP 0 242 626 B1 to provide a paste on the underside of the power devices. The paste contains a metal powder, preferably a silver powder, in a solvent. These pretreated power devices are connected with the aid of a pressure-sintering process to a contact layer of a flat form. The contacts of a flat form and the use of highly heat-conductive silver for contacting allowed the thermal transition between the power device and the contacting layer to be improved.
Further improvements in the cooling of power devices succeeded in the past with so-called DCB ceramics (DCB=Direct Copper Bonding). In DE 197 00 963 A1, it is proposed for this purpose on a ceramic substrate laminated on both sides with copper to solder the power devices onto the upper side of the DCB ceramic and to solder the underside of the DCB ceramic onto a metal plate acting as a leadframe. This metal plate passes the dissipated heat on to a connected cooling system. The upper copper layer of the DCB ceramic is structured (interrupted), whereby conductor tracks for contacting the power devices on their underside are formed. The further contacting of the power devices takes place on their upper side with bonding wires.
If it were desired to combine the benefits of the two processes described above, on the one hand the pressure-sintering of power devices onto a contacting layer and, on the other hand, the soldering of a DCB ceramic onto a metal plate for the further connection of the metal plate to a cooling system, at least two different process technologies would have to be used for the construction of a power device, that is pressure-sintering and soldering. To produce the connection to a cooling system, a further process technology is generally also necessary, since the metallic base plate of the power device is connected to the cooling system in a conventional way by a heat conducting paste. However, the use of a number of different process technologies in the constructing and connecting technique of power devices makes the process for producing the power devices complex and expensive.
On the basis of the prior art described above, the object according to the invention is to provide a power module with improved thermal behavior which is largely insensitive to thermal load changes.
According to the invention, this object is achieved by a power module of a construction which is connected merely by the pressure-sintering process. For additional improvement of the transient thermal behavior, the power devices are fitted with additional thermal capacitances.
An electronic power module according to the invention comprises at least one electronic power device, a DCB ceramic substrate, a heat sink and at least one additional thermal capacitance,
the electronic power devices being connected by a sintered layer on their underside to the upper copper layer of the DCB ceramic substrate,
the upper copper layer of the DCB ceramic substrate being structured into copper conductor tracks for the electrical contacting of the power devices,
the lower copper layer of the DCB ceramic substrate being connected by a sintered layer to a heat sink, and
the upper sides of the power devices being connected by a sintered layer to an additional thermal capacitance.
Further advantageous embodiments of a power module according to the invention are contained in the subclaims.
The following advantages are primarily achieved by the invention.
By dispensing with a metallic base plate, which is used in the conventional module construction of power modules, to mount the module onto a heat sink, the heat removal of the dissipated heat from the power devices into the heat sink is improved. The direct connection of the DCB ceramic to a heat sink improves the steady-state thermal resistance of the power module according to the invention in comparison with conventional constructions by up to 50%.
The reduced volume of the construction brought about by dispensing with a conventional base plate also has the effect of reducing the thermal capacitance of the construction, which is of significance for the transient thermal behavior. In order to utilize the improved heat transfers of the power module according to the invention optimally even under transient thermal loads, the power module contains additional heat capacitances on the surface of the power devices. As a result, briefly occurring peak values of the thermal dissipated power are buffered in the heat capacitances and, as a result, the transient thermal behavior of the power module is improved. By virtue of the heat capacitances arranged directly on the power devices, the transient thermal resistance of the power module is reduced by 25-30%, preferably halved.
The exclusive use of the pressure-sintering process as a connecting technique for the construction of the power module decisively increases its load changing resistance with respect to thermal load changes in comparison with a power module constructed by a conventional soldering technique. Under thermal load changes in the temperature range from minus 55xc2x0 C. to 125xc2x0 C., an increase in the load changing resistance by at least a factor of 12 can be achieved with a power module constructed by the pressure-sintering connection technique in comparison with a power module constructed by soldered connection.
By contrast with soft soldering processes, the sintered connection is produced as a consequence of a solid-state reaction. This has the consequence that the pressure-sintered connection, which is produced at a process temperature in the range of 215xc2x0 C. to 250xc2x0 C., can be used under the same or even much higher operating temperatures. On the one hand, subsequent mounting of components at the same process temperature is therefore possible, so that problems such as soldering with solders of different melting temperature no longer arise. On the other hand, the pressure-sintering technique can also be used for the construction of future generations of power devices on an SiC basis (silicon carbide basis), without modifications in the process technology being necessary for this.
A further advantage lies in the quality of a pressure-sintered connection. In particular, in cases of large surface areas, there are often voids (air inclusions) in soldered connections. The voids sometimes account for up to 50% of the soldered connection and consequently cause a rise in the thermal resistance. The pressure-sintered connection, on the other hand, can be produced with a small layer thickness of less than 30 micrometers in void-free quality even for large surface areas.